DRAINAGE METHODS FOR COUNTY ROADS. 51 
ditches filled Avith stone and provided with suitable outlets or by the 
use of tiling. Methods of constructing drains of either kind are de- 
scribed in another part of this bulletin. 
Where old road crusts or old base of broken stone, gravel, Telford, 
etc., are to be covered, the disposition of the old construction is im- 
portant. A highway engineer can commit no greater error than to 
abandon or destroy an old and well-compacted road crust if it can 
be used as a foundation for a new road. If the new construction is 
to be gravel or macadam, the lesser irregularities of the old surface 
can be filled with gravel or fine stone after the old surface has been 
swept thoroughly. These patches should be well watered, covered 
with screenings, and rolled. On a base so prepared the new road 
can be built. If a bituminous top is to be placed directly on the old 
crust, lean bituminous concrete can be used to excellent advantage. 
The old surface should be swept thoroughly, the depressed areas 
painted with the same bitumen used in the mix, and the patches laid 
and raked with care. A 3 or 4 foot straightedge with a handhole is 
convenient in bringing the patch to a true surface uniform with the 
old. The entire series of patches should be rolled and allowed to set 
for at least 12 to 21 hours ahead of the topping. 
If the new surface is to be wider than the old base or crust, it is 
more difficult to produce an even surface that will not settle at the 
edges. In many case 3 the only practicable disposition of the old crust 
is to loosen it and either waste the material or salvage it for the new 
top. Salvage methods usually are expensive. 
It should be borne in mind that when the old crust is left undis- 
turbed and simply evened up by the addition of new material, it 
is difficult to compact the new material as thoroughly as the old, and 
a tendency to uneven settlement of the subgrade will result. For 
this reason the greatest care should be used in making the prelimi- 
nary patches. Plate VII shows the results of neglecting this detail. 
In the premises the whole problem of making adequate use of old 
base is largely one of initial engineering. The grades should be laid 
with unusual care to take advantage of the old crusts, and where 
these are thin room should be left for necessary additional material. 
Where practicable it is always well to have the grading and drain- 
age structures for an improved road completed some time in advance 
of the pavement. This enables the roadbed to settle without doing 
any damage, and obviates the necessity for much of the tamping 
and compacting which otherwise would be necessary. 
COST OF SUBGEADE PEEPAKATION. 
The cost of preparing the subgrade of a road to receive the founda- 
tion or road crust depends to a very considerable extent on the degree 
